There is well known a writing instrument in which two-color ball point pen units are held in a tubular casing, and a cap holding a cam mechanism for engaging these units is rotated reciprocatingly through a certain angle around the axis of the casing, whereby said units are alternately shifted into a writing position or a storage position. However, in such a mechanism of the writing instrument, the cap can not be shifted in the axial direction, so that if one ball point pen unit is replaced by a mechanical pencil unit, the feed of lead by an axial shifting operation of the pencil unit cannot be effected.